1. Field of the Invention
This invention relates to a method of controlling the sintering temperature of metal particles. More particularly, this invention relates to a method of reducing the difference between the sintering onset temperature of the metal particles constituting a thick film conductor paste formed on a ceramic green sheet and the sintering onset temperature of the ceramic particles constituting the green sheet by preventing the sintering of the metal particles until a temperature close to the sintering temperature of the ceramic particles is reached.
The terms "sintering temperature" and "sintering onset temperature" are herein used interchangeably. They refer to the lowest temperature at which agglomeration of the particles of a given medium occurs by thermal means.
2. Description of the Prior Art
In the conventional technique of forming a multilayer glass-ceramic substrate carrier for mounting of semiconductor integrated circuit chips thereon, (unfired) ceramic sheets, referred to as green sheets, are prepared from a composition consisting of a high melting point ceramic powdered material such as aluminum oxide, titanium oxide, barium oxide or calcium oxide, a lower melting point glass material such as lead silicate, borosilicate, magnesium aluminum silicate or soda-lime glass or a combination thereof, an organic resin binder material such as polyvinyl butyral, vinylchloride acetate co-polymer, polyvinyl alcohol, polyvinyl chloride, polystyrene or polyethylene, a plasticizer such as dibutyl phthalate, butyl benzyl phthalate or dioctylphthalate and a solvent such as trichloro ethylene, methyl ethyl ketone, methylalcohol, cyclohexanone or toluene. Then, interlevel via holes at predetermined locations are punched in each green sheet followed by forming on the surface of each sheet a thick film electrical circuit pattern and filling the via holes thereon using a suitable conductive paste. The conductive paste is typically a metallizing paste of copper or another non-refractory metal wherein the predominant component is the metal. The green sheets are then stacked upon one another, in registry, in a laminating press ensuring signal path integrity from layer to layer after the stacking operation.
The laminated assembly is then subjected to a sintering cycle involving relatively high sintering temperatures and long periods of time to volatilize the organic binding material and subsequently reform the ceramic particles into a dense and substantially impervious multilayer ceramic substrate. A typical sintering cycle, which is disclosed in U.S. Pat. No. 4,234,367 issued to L. W. Herron, R. N. Master and R. R. Tummala and assigned to IBM Corporation, the present assignee, is illustrated in FIG. 1. Referring to FIG. 1, the process consists of initially heating the laminated assembly in a non-oxidizing ambient such as pure nitrogen or nitrogen plus hydrogen at a slow rate until a carbon oxidation temperature of about 780.degree. C. is reached. Upon reaching the burn-out temperature the ambient is switched to an ambient of H.sub.2 /H.sub.2 O at a volume ratio of about 10.sup.-4 and these conditions are maintained for about 6-8 hours. During this intermediate phase of the sintering cycle the hydrogen gas prevents oxidation of (i.e., serves as a reducing agent for) the metal particles constituting the thick conductor film and the water vapor enhances oxidation of the carbon and promotes the burn-out of the polymeric binder materials. In the final phase of the sintering cycle, the H.sub.2 /H.sub.2 O ambient is changed to nitrogen, with about 0.5-2.0 hour hold to remove entrapped or dissolved water with subsequent heating to a temperature of about 930-1000.degree. C. with about a 2 hour hold at this temperature for sintering and crystallization of the glass and ceramic particles constituting the green sheets.
To more fully delve into the sintering scenario of the conventional process, due to the relatively low melting point of the metal constituting the thick film conductor pattern in comparison with the melting points of glass and ceramic constituents of the green sheets, the metal particles undergo sintering with attendant shrinkage of the thick film pattern during the initial phase of the sintering cycle whereas the ceramic and glass particles undergo sintering during the intermediate and final phases of the sintering cycle along with their characteristic shrinkage. To explain the mechanism by which the metal particles will undergo sintering, attention is focused on a pair of metal particles in physical contact with each other at a low, pre-sintering temperature. As the temperature is raised, the metal particles will initially form a neck at their mutual contact point due to inter-particle diffusion. During this neck forming stage, there will be no shrinkage in the volume of the particles. As the temperature is raised further, however, the material from each particle will be transported into the other via the neck with eventual coalescence of the two particles into a single mass of a reduced dimension. The ceramic and glass particles constituting the green sheets will undergo sintering in a similar manner with a different shrinkage factor than the metal.
A typical relative percentage shrinkage of metal and ceramic materials due to sintering is illustrated in FIG. 2. As shown in FIG. 2, not only are the sintering onset temperatures of metal and ceramic different giving rise to a sintering temperature differential .DELTA.T, but also the sintering rates of these materials are quite diverse. .DELTA.S is the difference in total shrinkage between the metal and ceramic materials. As a result of this disparity in shrinkage rates, the multilayer ceramic substrate is prone to distortion introducing dimensional stability problems, i.e. proper contact of interlevel conductors is jeopardized. Another problem due to the shrinkage is that the locations of the top surface pads are not predictable with sufficient accuracy for high-speed automatic probing necessitating individually determining these locations which is not cost-effective in a high-volume manufacturing environment. Yet another problem due to the relatively low temperature sintering of the metal particles is the formation of one or more ceramic-metal separations within the interlevel via holes due to pulling away of the metal from the walls of the ceramic via hole. This is illustrated in FIG. 3 which shows a fired unitary ceramic structure 10 having two via holes 11 and 12 containing interlevel conductors 13 and 14, respectively. The separations in the via holes due to the pulling away of the metal discussed hereinabove are designated by numerals 15 and 16. As a result of these separations in the interlevel via holes, the multilayer ceramic package will be non-hermetic and is susceptible to penetration of chemical solutions and other hostile environments.
One method of delaying the sintering onset of the metal particles until at least the intermediate phase of the sintering cycle is to intersperse the metal particles in the thick film with a high melting point material such as molybdenum or aluminum oxide. However, this technique, among other disadvantages, undesirably lowers the conductivity of the conductor pattern, particularly when copper is utilized for the pattern, since a high densification of the metal cannot be achieved.
It is an object of the invention to reduce the difference in sintering temperatures of metal particles and glass-ceramic particles in the fabrication of a multilayer ceramic substrate without deleteriously affecting the electrical conductivity of the metal conductors.
It is another object of the invention to prevent the sintering of metal particles constituting the thick film conductor paste in the fabrication of a multilayer ceramic substrate until a high temperature close to the sintering temperature of the glass-ceramic particles is reached.
These and other objects of the invention will be apparent from the following description.